It is easy to presume that successful characterisation involves taking the reader to the heart, the inner self, of an imagined person. Once, however, "character" meant something different - something as much to do with the outer appearance of a person as with the inner being. "I have begun to acquire a composed, genteel character very different from the rattling, uncultivated one which for some time past I have been fond of," wrote James Boswell in his journal for 1762. Your "character" was what you presented to the world.

"Colonel Brandon's character... as an excellent man, is well established," says Elinor, the heroine of Jane Austen's Sense and Sensibility. "Character" here means reputation as well as individuality. It is social as much as psychological. One of the successes of Brick Lane is a triumph of characterisation that can be understood in just these terms. Chanu, the protagonist's husband, begins as someone the reader is invited to resent. His marriage to Nazneen has been arranged. She does not know him before she becomes his wife. He has been foisted on her and us. We are given plenty of reasons to be irked by him. He is self-important; "rolls of fat" hang low from his stomach; he lectures his children; he makes his wife cut his corns.

Yet slowly he becomes human. His incongruous quotations from Open University philosophy modules, his pronouncements on the "village" habits of his fellow Bangladeshis, his sociological explanations of British culture: these all become sympathetic without ceasing to be absurd. His intellectual generalisations and ill-founded ambitions are irrepressible. Those older expectations of "character" are a guide to Monica Ali's method with Chanu. For she does not inspire sympathy by giving us access to his thoughts or fears. We see his private self - his behaviour within the family home, even in the marital bed - but we keep meeting him only through his outward habits and his would-be wise pronouncements.

He lives partly in "his own private world of theory and refutation, striving and puzzlement". His sense of being educated is his defence against disappointment. "It is lucky for you that you married an educated man. That was a stroke of luck." A minicab driver living on a Tower Hamlets council estate, he talks himself into optimism. "Of course, when I have my Open University degree then nobody can question my credentials." He has a cupboard full of certificates from various courses and examinations. He is going to get promotion. He is going to return to Bangladesh to make a fortune from a soap business.

Any novelist can simply tell us what a character thinks. Ali has decided to do so with only one of her characters, Nazneen. To the very end of the book we stay inside her head. We know for sure only her feelings about her marriage, only her sense of how she and her husband avoid each other's thoughts. This commits the novelist to a difficult trick of characterisation with her husband: bringing Chanu alive from the outside. We must slowly sense him through the idiosyncrasies of his presentation of himself.

There is a wonderful comic episode that captures this, where Chanu takes Nazneen and their two daughters on a trip round tourist London, ending at Buckingham Palace and a picnic in St James's Park. In ridiculous mid-calf shorts and baseball cap he enthuses about the history of building and monuments (history being another of his areas of self-declared expertise). He reduces his family to hysterical laughter. "He swelled with pride at how marvellously he had managed the day. 'It is a lot of fun'." He does not get the joke, yet his declaration banishes embarrassment or ill will.

Ali never lets Chanu reveal his inner self directly. Indeed, the plot of Brick Lane turns on our uncertainty about his feelings. Near the end, are we to guess from his behaviour that he knows that his wife is unfaithful? Even Nazneen is not allowed to be sure that he has guessed at her affair with Karim ("how many times had she willed him to know? He would not yield to her and yet he must know"). The novelist respects the character enough to allow him to keep things to himself.

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